Heretofore it has been proposed in U.S. Pat. No. 4,213,040 to encode digital information in rows and columns on a record carrier. Data is read from the record carrier by movement of the record carrier on an X axis and rotation transport mechanism which is operable to make skew corrections.
Also heretofore various optical encoded data record carriers and optical "writers" and "readers" have been proposed. For example, in U.S. Pat. No. 3,549,897 there is disclosed an absolute electro-optical encoder for indicating the angular position of a shaft. The encoder includes a stationary disc and a rotary disc, the discs having concentric tracks with transmissive and non-transmissive portions which are binarily related. Light passing through the discs is picked up by certain selected combinations of photocells for indicating the position of one disc relative to the other disc.
In U.S. Pat. Nos. 3,501,586; 3,624,284; 3,885,014; 3,795,902; 3,806,643; 3,891,794; 4,090,031; and 4,163,600 issued to J. Russell, various "writers" and "readers" are disclosed for "writing" digital data on a spiral track and for "reading" digital data from the spiral track. In the optical encoding and decoding systems described in these patents opaque spots on the track correspond to logic 1 bits of binary data and transparent spots on the track correspond to logic 0 bits of binary data. Also, larger synchronization spots are provided at different places along the track.
An apparatus for scanning a data record medium is disclosed in U.S. Pat. No. 3,898,629 wherein binary digital information is recorded in the form of data along a circular arc and a plurality of such circle arcs of data information are arranged tightly adjacent each other.
In U.S. Pat. No. 3,919,697 there is disclosed a data record having track lines which may be separate parallel tracks or may be a single series track of the spiral or raster type.
In U.S. Pat. No. 3,983,317 there is disclosed an astigmatizer for a laser recording and reproducing system. In this system concentric circular tracks are formed in a thermoplastic record or disc by burning selected holes through the disc with a laser. The laser is "on" while the disc is being rotated a short distance to form an elongate data information bit in the track. Then, in reading the data the laser beam or spot is elongated in a direction transverse to the direction of the track with an astigmatizer unit so that a small elongate beam of light with an axis extending transverse to the axis of the track and of the elongate opening therein is used to read the opening.
In U.S. Pat. No. 4,094,010 there is disclosed an optical multi-channel digital disc storage system. Data is stored on a spiral information track and holes corresponding to the information data are burned into the material of the disc by an information radiation beam.
U.S. Pat. No. 4,094,013 discloses an optical storage disc system with disc track guide sectors wherein the data tracks are spiral shaped turns or concentric turns on the disc. The data stored is again in the form of holes burned into the disc.
U.S. Pat. No. 4,209,804 discloses a record carrier containing information in an optically readable radiation reflecting information structure. With the record carrier of this patent, data is stored in a spiral track on a disc in the form of information areas comprising pits pressed into the record carrier surface or hills projecting from the record carrier surface. According to the teachings of this patent, the depth of the pits or the height of the hills is constant and so is the width of the information areas and intermediate areas at the level of the plane of the lands. Then the information to be conveyed by the record carrier is contained in the variation of the structure of the areas in the tangential direction only. More specifically, the information areas are substantially V-shaped, the phase depth of each information area having one value between 100.degree. and 120.degree. and the angle of inclination between the walls of the information areas and normal to the record carrier are substantially constant and have a value between 65.degree. and 85.degree..
As will be described in greater detail hereinafter the coded data on the record carrier and the method for encoding same of the present invention provide a relatively simple and yet very effective means for making the data record carrier, and particularly the data recorded thereon, highly tolerant of errors such as "burst errors" where one or more spots of data on the data record carrier are obliterated.
The data is recorded on the data record carrier in arcuate nested tracks, each track comprising a stream of data bits. The stream of data bits includes an encoded front track address and encoded back track address with coded groups or so-called Hamming encoded groups of data bits (hereinafter simply Hamming groups) therebetween each of which can comprise 96 or 105 data bits. Error correction is provided by including coded or Hamming group data words which include a data byte and a field of parity bits, typically an eight bit data byte followed by a four bit parity field. Also a cyclic redundancy checksum byte is provided in the group as a check of the data. Further the data in each group is multiplexed.
Moreover, one or two redundant groups are provided in each track for recreating one or two missing groups. Also, a cyclic redundancy checksum field is provided in each track for further error detection.
With this encoding, the data included in the eight bit data bytes can be checked with the parity field bits and if there is an incorrect check, then the incorrect or missing bits can be recreated. Likewise, the cyclic redundancy checksum checks the totality of the data bits in each group to see if the cyclic redundancy checksum agrees with the data retrieved. Then the larger cyclic redundancy checksum field in each track can be used to check all the data in the three to twenty one groups in each data track for accuracy of retrieval and the redundant group or groups can be used to recreate missing data.
The multiplexing serves to stretch out the data so that the bits forming one data byte are spread out over the stream of data bits in each track. As a result, if there is a "burst error", hopefully it will result in the deletion of only one or two bits from each data word in a group enabling the encoded error correction information in the parity field to be utilized in recreating the missing data bits.